The present invention relates to the improvement of endosseous implants for living bodies. More particularly, it relates to an improved implant for a bone, a joint and tooth root comprising a metallic base material and a layer of a hydroxyapatite formed on the base material.
The so-called implantology which comprises insertion of artificial materials such as artificial organs, artificial blood vessels, artificial joints, artificial bones and artificial tooth roots into living bodies so as to recover lost parts of living bodies or their functions has been in the limelight in recent years. It is said that a trial of implantation goes back to ancient times. Particularly in these ten-odd years, a huge number of treatments by implantation have been performed on bones and tooth roots to afford good results in the remedy of the defects or recovery of functions thereof. However, an artificial bone or tooth root which can satisfy the necessary requirements as the material for living bodies, i.e. affinity to living bodies, safety, excellent mechanical strength (e.g. pressure resistance or impact strength) and durability, has not yet been obtained.
As metallic materials conventionally used for preparation of artificial bones or tooth roots, mainly cobaltchromium alloys, stainless steel, titanium and tantalum are exemplified. On the other hand, as ceramic materials, titanium oxide, aluminum oxide, calcium oxide-aluminum oxide, aluminum oxide-silicon dioxide glass, silicon dioxide-sodium oxide-calcium oxide-phosphorus pentaoxide glass (bioglass) or carbon materials are exemplified, and bioceramics such as apatite [Ca.sub.5 (Cl, F) (PO.sub.4).sub.3 ] have been recently taken note of.
Although metallic materials are excellent in mechanical strength, particularly in impact strength, they are faulty in the affinity to tissues of living bodies. For example, when a metallic implant is used, metal ions are dissolved out therefrom in living bodies and affect a toxic action to bone cells around the implant. Furthermore, the bone-formation is obstructed probably because of too large a thermal conductivity of the metallic implant. Among the metallic materials, tantalum is particularly superior in a corrosion-resistance and hence has been employed as fixing plates for skulls or fractured parts of bones and implants for jawbones, but this metal is difficult to be processed. To the contrary, ceramic materials show generally a good affinity to bones, and bone tissues penetrate into fine pores of the ceramic materials to afford a strong fixation, without reaction between the ceramic material and the tissue. Besides, they are also excellent in durability, that is, they are resistant to corrosion decomposition. But on the other hand, they possess a poor impact strength. Moreover, these artificial materials are entirely heterogeneous to the components of the hard tissues such as bones or tooth roots, and hence, they are merely a dead material in the living bodies, while they do not show a toxic effect thereto. Then, there is taken note of the apatites which are similar to the components of bones or tooth roots. Particularly, hydroxyapatite, which is a main component of the inorganic materials in the hard tissues such as bones or tooth roots, is absorbed into the living body and is simultaneously replaced by the newly grown bone in the host (i.e. the human body to be implantated), and therefore, the hydroxyapatite has a particularly excellent affinity to the tissues of living bodies. However, the hydroxyapatite has also a defect, i.e. a poor impact strength, much like other ceramic materials.
As a result of the present inventors' studies, it has now been found that desirable implants having a sufficient impact strength and hence an excellent breakresistance while retaining the advantages of ceramic materials and hydroxyapatite as above-mentioned can be obtained by combining a metallic base material and a layer of hydroxyapatite.
An object of the present invention is to provide an improved implant having excellent mechanical strength and an affinity to living bodies.
Another object of the present invention is to provide a method for improving metallic implants by coating a hydroxyapatite or a mixture thereof with ceramics on the surface of a metallic base material to form a layer of the hydroxyapatite.
A further object of the present invention is to provide an improved implant for various bones including tooth roots which exhibits good durability.
These and other objects of the present invention will be apparent from the following description.
The implant of the present invention comprises a metallic base material having an excellent mechanical strength and a coating layer of hydroxyapatite which is formed by thermally spraying hydroxyapatite powder (I), a mixture of hydroxyapatite powder and ceramic powders (II) or ceramic powders and subsequent hydroxyapatite powder (III) around the outersuface of the metallic base material.
The present invention will be hereinafter explained in detail with reference to the accompanying drawings, which are merely illustrative of an embodiment of the invention but the present invention is not limited thereto.